orm that it can lie almost
unseen upon the floor of the ocean. The eye on the under side of the
body would thus be useless, but a glance at a sole or plaice in a
fishmonger's shop will show that this eye has worked upward to the top
of the head. Was the eye shifted by the effort and straining of the
fish, inherited and increased slightly in each generation? Is the
explanation rather that those fishes in each generation survived and
bred which happened from birth to have a slight variation in that
direction, though they did not inherit the effect of the parent's effort
to strain the eye? Or ought we to regard this change of structure as
brought about by a few abrupt and considerable variations on the part
of the young? There you have the three great schools which divide modern
evolutionists: Lamarckism, Weismannism, and Mendelism (or Mutationism).
All are Darwinians. No one doubts that the flat-fish was evolved from an
ordinary fish--the flat-fish is an ordinary fish in its youth--or that
natural selection (enemies) killed off the old and transitional types
and overlooked (and so favoured) the new. It will be seen that the
language used in this volume is not the particular language of any
one of these schools. This is partly because I wish to leave seriously
controverted questions open, and partly from a feeling of compromise,
which I may explain. [*]

* Of recent years another compromise has been proposed
between the Lamarckians and Weismannists. It would say that
the efforts of the parent and their effect on the position
of the eye--in our case--are not inherited, but might be of
use in sheltering an embryonic variation in the direction of
a displaced eye.

First, the plain issue between the Mendelians and the other two
schools--whether the passage from species to species is brought about
by a series of small variations during a long period or by a few large
variations (or "mutations") in a short period--is open to an obvious
compromise. It is quite possible that both views are correct, in
different cases, and quite impossible to find the proportion of each
class of cases. We shall see later that in certain instances where the
conditions of preservation were good we can sometimes trace a perfectly
gradual advance from species to species. Several shellfish have been
traced in this way, and a sea-urchin in the chalk has been followed,
quite gradually, from one end of a genus to the other